This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-146714, filed May 16, 2001; and No. 2001-260304, filed Aug. 29, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an ink-jet recording apparatus using a plurality of ink-jet heads.
2. Description of the Related Art
Ink-jet recording apparatus using a serial type ink-jet head are currently in the main stream. This is because they have advantages including ease of maintenance due to the small number of ink ejection nozzles that the ink-jet head has and low manufacturing cost.
On the other hand, however, a serial type ink-jet head has disadvantages including that the width over which it can apply ink for printing in a single scanning operation is limited because of the limited number of ink ejection nozzles it has. Therefore, the ink-jet head has to be made to scan for a number of times before printing an entire page so that it takes a long time for printing a page.
Meanwhile, a long ink-jet head formed by arranging a large number of ink ejection nozzles can apply ink over a large width for printing in a single scanning operation because of the large number of ink ejection nozzles it has. Therefore, the ink-jet head can print an entire page with a reduced number of scanning cycles and hence is adapted to realize high speed printing.
However, a long length type ink-jet head has disadvantages including difficulty of manufacturing due to the large number of ink ejection nozzles it has. Therefore, the manufacturing yield is low and the cost is high.
There is known a technique of forming a long length head by combining a plurality of ink-jet heads each of which has a relatively small number of ink ejection nozzles for the purpose of dissolving these problems.
FIG. 15 illustrates a known ink-jet recording apparatus comprising a combination of a plurality of ink-jet heads.
As shown, an ink-jet head 2 is fitted to an end of a holding substrate 1 at a lateral side thereof while another ink-jet head 3 is fitted to the opposite end of the holding substrate 1 at the other lateral side thereof to form a long length head. A sheet conveyor belt 4 is arranged below said ink-jet heads 2 and 3, spaced apart from the heads 2 and 3. The belt 4 conveys the recording sheet 5, making it pass below the ink-jet heads 2 and 3 in a direction away from the viewer of FIG. 15.
FIG. 16 illustrates the positional relationship of the ink ejection nozzles 21, 22, 23, . . . , 2n-2, 2n-1, 2n of the ink-jet head 2 and the ink ejection nozzles 31, 32, 33, . . . , 3n-2, 3n-1, 3n of the ink-jet head 3 that can be observed when said ink-jet heads 2 and 3 are viewed from above.
The ink ejection nozzles of each of the ink-jet heads 2 and 3 are arranged at a pitch P. The most closely located ink ejection nozzles 21 and 3n of the ink-jet heads 2 and 3 are also separated from each other by a distance equal to P.
The ink-jet head of this ink-jet recording apparatus can be used to print over a large width just like a long length ink-jet head by regulating the moving speed of the recording sheet 5 relative to the ink-jet heads 2 and 3 and the timings of ejecting ink of the ink-jet heads 2 and 3.
More specifically, in this ink-jet recording apparatus, is adapted to print characters for a line by means of the ink-jet head 2 prints the first half of a line, and the ink-jet heat 3 prints the remaining half of the line after a predetermined time. With this arrangement, the apparatus prints a line of characters as if it printed by means of a single head.
However, with this ink-jet recording apparatus, there is a time lag before the ink-jet head 3 starts printing after the end of the printing operation of the ink-jet head 2 for the line. Then, the ink ejected from the ink-jet head 2 and the ink ejected from the ink-jet head 3 behave differently in terms of the extent of ink absorption of the recording sheet 5 and the extent of ink drying.
Therefore, there can occur a phenomenon that ink layer of the part of the image on the recording sheet formed by the ink ejected from the ink-jet head 2 and that of the part of the image formed by the ink ejected from the ink-jet head 3 differ from each other along the boundary thereof.
In other words, the ink layer 6 of the image formed by the ink-jet heads 2 and 3 on the recording sheet 5 can show a recess a at the connecting portion of the image as shown in FIG. 17.
As the recess a is formed in the ink layer 6, the recess a appears as a stripe-shaped deviated density zone when the entire image is viewed from above. The defect of the image due to such a stripe-shaped deviated density zone is conspicuous because it is produced as a linear defect.
The object of the present invention is to provide an ink-jet recording apparatus that can form an ink layer uniform in thickness even at a junction between a part printed by an ink-jet head and another part printed by another ink-jet head.
According to an aspect of the invention, there is provided an ink-jet recording apparatus that comprises a plurality of ink-jet heads, each having a plurality of ink ejection nozzles arranged at a predetermined pitch. The ink-jet heads are arranged at predetermined intervals in the direction in which a recording medium is moved. The ink ejection nozzles of each head are arranged in a line crossing the direction in which the recording medium is moved. The ink-jet heads are driven at different times so that the dots they print on the medium align with one another, forming a straight line. The ink-jet heads are so arranged that the distance between the outermost nozzle of one head and the adjacent nozzle of the next head is shorter than the predetermined pitch at which the nozzles of any head are arranged.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.